1. Field of the Invention
This invention relates to the production of highly oriented strontium ferrites which form excellent ceramic permanent magents.
2. The Prior Art
The strontium ferrites produced by the instant process are described in U.S. Pat. No. 3,884,823, issued May 20, 1975, on application Ser. No. 183,896, filed Sept. 27, 1971.
Metal ferrites having a formula MO.multidot.nFe.sub.2 O.sub.3 wherein M is barium or strontium and n has a value of about 6, are known to yield permanent magnets.
Conventionally ferrite magnets are prepared by an involved multi-step process. First, the two individual metal oxides are calcined and reacted together to form a solid ferrite body. The resulting ferrite body is then ground up and ball milled into a fine powder. This powder is slurried in liquid and the slurry is placed in a magnetic field which orients the individual ferrite particles. While still aligned in the magnetic field, the slurry of particles is wet-pressed to form a compacted solid of oriented particles. This solid is dried and then thoroughly sintered. Finally it is magnetized to give the final ceramic magnet product. This process has several disadvantages. In addition to being complicated, it also does not produce magnets having optimum magnetic properties (remanences and coercive forces), since it does not produce a fully oriented material made up of uniform small crystallites.
Both a desirable small particle size and a full orientation cannot be achieved using conventional techniques for two reasons: (1) orientation during wet-pressing requires large (2 micron or larger) particles to be fully effective, thus either orientation or particle size with their associated magnetic properties must be sacrificed; and (2) it is impossible to both restrain grain growth during sintering and achieve desired high densities.
It has recently been proposed to effect crystallite orientation by hot working a conventionally prepared solid ferrite. In "Magnetic-Crystallographic Orientation Produced in Ferrites by Hot Working", R. M. Haag, Annual Report Mar. 15, 1968 - Mar. 14, 1969, concerning Office of Naval Research Contract N00014-68-C-0364, issued on Mar. 14, 1969, the hot forging of barium ferrites only is disclosed. This report discloses the well-known ball milling technique to produce powders, but fails to note any other methods such as chemical precipitation to produce sub-micron particles. Also, this report is silent as to magnetic properties of the resulting barium ferrites and suggests that further work should be conducted.
Another report, "Magnetic-Crystallographic Orientation Produced in Ferrites by Hot Working", R. M. Haag, issued on Mar. 14, 1970, covering work on the above-noted contract from Mar. 15, 1969 to Mar. 14, 1970. This report, like the previous report discloses the use of wet ball milling techniques and is essentially silent regarding the magnetic properties, grain size or orientation of the resulting ferrites.
In a third, and concluding report of the same title and author, issued Mar. 14, 1971, covering work on Office of Naval Research Contract N00014-70-C-0278, from May 5, 1970 to Mar. 14, 1971, strontium ferrites are mentioned. However, the use of wet ball milling techniques are used and the magnetic properties, particularly the balance of magnetic properties of the resulting strontium ferrites are still too low.
The process of the present invention, while somewhat more complicated than known heat/pressure orientation processes, produces a product which forms finite ceramic magnets that are superior in overall magnetic characteristics to conventionally prepared ceramic magnets such as those prepared by the above-noted "Haag" process.